1. Field of the Invention
This invention relates to a method and apparatus for processing an audio signal and more particularly to a technique for permitting varying degrees of distortion to the audio output for a guitar or other musical instrument without causing any substantial change in the volume of the output signal and in the treble and bass content of this signal.
In playing various types of electronic musical instruments, such as for example electric guitars, a desirable acoustic effect can be achieved for some types of music by the controlled distortion of the instrument output. An undistorted signal is generally referred to as a "clean signal". Distortion is may be achieved by increasing the gain of the signal as the signal is applied to a distortion amplifier, overdriving the amplifier so that a portion of the wave-form is clipped, or by clipping in other ways, some of which may result in a reduced volume output with increasing distortion. Depending on the desired musical effect, distortion may vary from very little clipping or distortion to heavy distortion where most of the wave form is clipped.
While distortion devices have been on the market for many years, existing devices of this type have a number of limitations. First, because of the manner in which distortion is achieved through use of either a distortion amplifier or other forms of clipping, as distortion on a signal is increased, the output volume of the signal is variably increased. This variation in volume is generally undesirable and is particularly undesirable when the instrument is being played as part of a band or as a backup for a singer where it can adversely affect the balance of the group. In the past, a musician might compensate for this volume change by manually adjusting the output volume when he changed the degree of distortion. However, having to adjust two controls during a live or studio performance is difficult and it is even more difficult under these circumstances to achieve anything resembling a uniform output volume level. While devices exist which permit a particular preset volume level for a particular distortion setting, and U.S. Pat. No. 4,752,962, entitled "Audio Processing Circuit" issued Jun. 21, 1988, teaches a circuit which detects output volume and uses the detected output volume to perform compensations, there is no system currently on the market which automatically compensates for volume changes solely as a function of a selected distortion level over the full distortion range of the distortion device. Operating solely in response to the selected distortion is simpler and less expensive than the circuit shown in the patent and may eliminate noise and distortion caused by spurious volume changes or to rapid response thereto.
Another problem with distortion devices is that the distortion tends to alter the harmonic content of the output signal, and in particular to make the signal brassier or more treble. There may also be some increase in the perceived bass content as distortion increases. Such changes in harmonic content and spectral density may also adversely affect balance in a band setting and are thus also undesirable; and such variations are also very difficult to compensate for during a live performance. Again, while a preset may be possible for a single distortion setting, and U.S. Pat. No. 4,752,960 teaches some tone compensation based on a detected volume level, no technique or apparatus is currently available which automatically compensates for such spectral and harmonic variations over the full range of the distortion device solely as a function of the selected distortion level.
Finally, it is common to use a compressor circuit at the input to the distortion device. A compressor is basically an amplifier, the gain of which varies as a function of the amplitude of the signal applied thereto. Since rapid changes in this gain can cause distortion of a clean audio output, the release time, which is the time required for a change in gain to occur as a result of an increase in audio input volume level, it is generally relatively long for such compressors. Typically, the release time is in the range of one half to one second for a olean signal. However, the distortion caused by a shorter release time is not a problem for a signal which is undergoing relatively heavy distortion, and it is desirable that the release time of the input compressor be reduced when the distortion device is operating with a relatively high degree of distortion. Existing distortion devices do not alter the release time of the input compressor based on the degree of distortion. In most systems it would be difficult for the musician to achieve this desirable affect.
It is therefore desirable that an improved method and apparatus be provided for distorting an audio signal such as that generated by an electronic musical instrument which permits such distortion to be achieved without any substantial change in the volume, base content or treble content of the audio output and in particular that these capabilities be automatically achievable in response only to the musician selecting a desired level of distortion.